bundle Decal, the open-source ReaPrime skin

[ChampionDesigns]

Summary

Lists Decal in skin_sources.json, so Decaid bundles it the way it already bundles six other
third-party skins.

Base: main. Independent. Four lines.

{
  "type": "github_release",
  "repo": "ChampionDesigns/decal"
}

What this is and is not

assets/bundled_skins/ is gitignored — Decaid does not commit skins. It reads this manifest and
fetches each repo's latest release. So this PR adds no skin code, only a source to fetch from,
exactly as allofmeng/streamline_project, tadelv/passione, rotium/OverDose,
giladger/Beanie, NilsBruch/NSX and Sabotage1/WorkFlow-Skin already are.

Whether Decaid should ship Decal is a product decision, not a technical one. The change itself
is trivial; the question is whether you want it in the box.

Linked Issue

N/A

Verification

• flutter analyze — clean.
• flutter test — full suite 3891 passed / 1 skipped, run against current main on
  5 Sep 2026.
• flutter test — test/webui_support 26 passed.
• skin_sources.json parses; 8 entries, Decal among them.
• ChampionDesigns/decal is public and has release v0.1.0 with decal-0.1.0.zip, matching
  the single-zip asset shape the fetcher already consumes from passione-v0.9.5.zip.
• Verified on hardware. Decal is bundled in the Decaid-Canary build Ben runs, fetched from the
  same public release this entry points at.

Impact

• User-visible: Decal appears in the bundled skin list on a fresh install.
• Download size: one more skin fetched at first run — decal-0.1.0.zip is about 1.0 MB.
• Compatibility: additive. Existing installs are unaffected until they refresh their skin list.
• Security: the skin is fetched from a public GitHub release, the same trust path as the other
  six.

Contributor Responsibility

AI-assisted development is allowed. The submitter remains responsible for the submitted work.

• I have reviewed and understand all changes in this PR and take responsibility for their correctness, security, behavior, licensing, and provenance, including any AI-assisted or AI-generated work.

[tadelv]

Thanks. I don't want to add new bundled skins to Decaid at the moment. Our highest development priority is the generic device-driver/plugin infrastructure, and until that work settles I'm keeping new skin and plugin bundling out of the core distribution.

Decaid already has a native installation path for third-party skins:

Launcher → Skins → + → GitHub Release

From there a user can enter ChampionDesigns/decal and install/update Decal without it being added to Decaid's bundled skin sources.

I think the best way forward is therefore to document that installation path in the Decal README rather than bundle Decal in Decaid. The current README still points users at the raw /api/v1/webui/skins/install/url API; replacing that with the native UI instructions would make Decal straightforward to install without requiring any Decaid change.

I'm leaving this PR open for now, but I don't plan to merge skin-bundling changes while the generic-driver work remains the priority.